What Animals are Marked and Recaptured?

It is often necessary for biologists and wildlife managers to estimate the population size of wild animal species. Knowing whether population is increasing or decreasing is integral to conservation management, and essential in cases of threatened or endangered species. However, it is often impossible to obtain a complete count of animals in their natural environments. As a result, various methods for estimating population size have been devised.

Methods for Estimating Population Size

The simplest method for estimating population size is counting individuals by various means in random designated areas called quadrats. Another, known as the transect method, involves method involves counting individuals in larger areas in which quadrats are placed at fixed intervals.¹

The Mark-Recapture technique is a method that falls between these two.² This involves marking a subset of a population, followed by later counts on the relative numbers of marked (recaptured) and unmarked (not previously captured) individuals. This method is considered to be more precise than an arbitrarily-derived census in which no animals are marked, but involves less time and expense than a comprehensive marking and tracking effort.

What is Mark-Recapture?

The Mark-Recapture method is used for all manner of wildlife species, and is particularly effective with populations of smaller animals, such as birds, fish and small invertebrates. It is currently the most popular way of estimating the size of a population, and is widely used by fish and wildlife managers to estimate population sizes before fishing or hunting seasons.²

Mark-Recapture involves capturing and marking a number of individual animals in a population and returning them to that population. Groups of animals are subsequently captured, which necessarily include some of those that were captured initially. The ratios that arise from the captured and recaptured animals form the basis for estimating the size of the population at the time of the marking and release. Marking materials can consist of a wide range of materials, such as paint, metal bands, metal clips, collars, ear tags and radio tags.

The design of a Mark-Recapture study is important, and helps to determine which model is appropriate for estimating population size.¹ One assumption that is made when estimating the size of population via Mark-Recapture is that the population is closed. A closed population remains constant in size and composition throughout a study, whereas an open population is subject to animals leaving and entering the population through births, deaths, emigration, and immigration. Thus, analyzing data of open populations is often complicated due to animals leaving and entering populations.

Interested in learning more about safe capture? The San Diego Zoo now offers courses in safe capture techniques and best practices. Learn reliable, safe, and effective techniques for the species you work with and the scenarios you encounter!